This invention relates to distribution line powered switchgear controls. More particularly this invention relates to circuits which rapidly charge a power capacitor in such controls to enable them to function reliably.
A network of power distribution lines must respond to normal variations of load requirements and abnormal fault conditions to maintain service to the greatest number of customers in an economical manner. To that end a variety of switchgear is used to vary the interconnections among the distribution lines. By necessity much of the switchgear is located far from power distribution centers and must have standalone capability. The types of switchgear employed include remotely actuated switches, sectionalizers and reclosers.
Remotely actuated switches are controlled by a power distribution center and may include indicating means which signal their state or the existence of a fault on a distribution line.
Sectionalizers are similar to remotely actuated switches but lack their ability to interrupt fault current. They are actuated after a fault interrupting device has acted to interrupt their source.
Reclosers sense faults on a distribution line and open to interrupt a fault a limited number of times within a short time interval. Most faults result from a temporary condition such as a branch brushing against a line. Once a fault resulting from a temporary condition is interrupted, the fault is often cleared until an initiating condition reoccurs. Faults resulting from a more permanent conditions such as a downed line are prevented from reoccurring by locking out or opening the contacts of a recloser until the cause of the fault is eliminated.
Switchgear should be located and coordinated in such a manner as to minimize an area of power outage due to a fault condition.
Most switchgear requires considerable power for reliable operation and it is difficult to supply remotely located switchgear and switchgear controls with power sources which are both reliable, under normal and abnormal conditions, and inexpensive. Much switchgear is operated by springs which are tensioned to store energy under normal distribution line conditions. An example of spring operated switchgear is disclosed in U.S. Pat. No. 4,293,834 to Date et al. Switchgear controls likewise have some energy storage means to enable them to operate when their associated distribution line is disconnected.
Switchgear controls in a quiescent state, when they are not initiating a opening or closing of the distribution line switches, have relatively low power requirements. Typically tens of milliamperes will suffice to keep the control reliably functioning in a quiescent state. However when the switchgear control enters an active state to initiate opening or closing the distribution line switches, the power required rises dramatically. Typically the control requirements will rise an order of magnitude or more and, amperes may be required in the active state. These increased power requirements must be furnished by the energy storage means which must either be maintained at a high level of storage or be very rapidly replenished.
The switchgear mechanisms which positions the distribution line switches take a relatively long time to completely respond to a switchgear control signal. Those mechanisms which are oil filled take about a half second to completely respond. If the energy storage means can be maintained at or restored to supply operating power within a half second, the switchgear control could initiate a close operation and then open immediately on the closing of distribution line switch. Restoring the switchgear control energy storage means within a half second would be considered adequate even when the control is associated with mechanisms which respond much faster than a half second.
Controls for switchgear, which initiate movement of the mechanism, are often battery operated with the battery being charged under normal operating conditions. An example of battery operated switchgear is disclosed in U.S. Pat. No. 3,116,439 to Riebs.
Some switchgear controls are operated from power capacitors which are charged by the distribution line under either normal or faulted conditions. An example of a power capacitor operated switchgear control is disclosed in U.S. Pat. No. 4,027,203 to Moran et al which obtains its power from distribution line currents. Another example of power capacitor switchgear control is disclosed in U.S. Pat. No. 4,352,138 to Gilker which obtains its power from the voltage on the source side of the line.
Each energy storage means to provide switchgear control operating power has advantages and disadvantages. Battery operated switchgear controls are expensive and may become inoperative due to battery or battery charging circuit failures particularly when fault occurrences extend over a long period. A number of approaches to minimize failures exist. One approach disclosed in U.S. Pat. No. 3,381,176 to Riebs et al. disconnects a battery after a successful closing operation or after a predetermined time after the switch opens.
Capacitor powered switchgear controls avoid the expense of a battery but, usually will not maintain a state of charge as long as a battery and become uneconomical if the energy stored approaches that typically found in batteries. Often a single operation will severely deplete the amount of energy stored in a power capacitor of moderate size and cost. Depending on the size of the power capacitor and and loading in the active state, the voltage of the power capacitor and the control bus may fall on the order of five or more volts. If current transformers alone were used to restore operating voltage an undesirable period of unreliable control operation would result. A potential transformer on the source side of the switchgear may be employed to maintain charge on the power capacitor. However such approaches as disclosed in U.S. Pat. No. 4,352,138 to Gilker requires the addition of a potential transformer and in that invention two power capacitors. The second power capacitor operates to supply power in the event that the preferred potential transformer source of capacitor power and a secondary battery source both fail. Current transformers, necessarily present to monitor current in the distribution line, charge the second power capacitor at the expense of accuracy when charging occurs. The use of multiple sources of power to achieve greater reliability is not unusual and other approaches exist.
The use of current transformers to charge a power capacitor is particularly economical when the current transformer must be present to monitor distribution line current. However there are difficulties associated with the use of the monitoring current transformers to charge a power capacitor. An additional dilemma involves the speed with which the capacitor is allowed to charge.
If the power capacitor is allowed to charge with great rapidity, sufficient current is drawn to adversely affect the accuracy of the current monitoring function of the control. A rapid charging of the second power capacitor in U.S. Pat. No. 4,352,138 to Gilker only occurs when the primary and secondary sources have failed. Additionally U.S. Pat. No. 4,352,138 to Gilker discloses a circuit to prevent closing the distribution line switches when insufficient power exists to allow the control to operate the switchgear to an open position.
Alternately the power capacitor may be placed in series with a test resistor which monitors phase currents as disclosed in U.S. Pat. No. 4,393,431 to Gilker and U.S. Pat. No. 4,131,929 to Moran. However the rate of charging the power capacitor is limited by the test resistor.
Alternately, the power capacitor may be a charged and maintained at a more moderate rate by inserting a moderate resistence in the charging path with a less serious affect on the current monitoring function. However during the period the power capacitor is being charged to an appropriate level, the control should be inactivated to prevent unreliable operation. U.S. Pat. No. 4,027,203 to Moran discloses one method of inhibiting control operation while the power capacitor is being charged to an appropriate level. Similar dilemmas exists when the preferred source of supply for the power capacitor is either a potential transformer or a battery.
When a potential transformer is the preferred source the designer must chose between providing a high or a low resistance path to the power capacitor. A low resistance path will rapidly charge the capacitor but results in vastly oversizing the transformer for an intermittent demand of the power capacitor. If the transformer is not oversized an early failure will result from overstressing the transformer to continuously supply what should be an intermitted demand. Alternately if a high resistance path is chosen the power capacitor is unable to furnish sufficient power for reliable switchgear control operation until the power capacitor has been charged for a longer period.
If a battery is chosen as the preferred source for capacitor charging the dilemma remains. A high resistance charging path results in longer periods of unreliable switchgear control operation while the power capacitor is being charged. A low resistance path will more rapidly exhaust the battery and require batteries of great size.
Current transformers typically used in switchgear controls to monitor phase currents have ratios between the primary and secondary currents on the order of 1,000 to one. If 100 amperes is flowing through the primary of such a phase transformer than one tenth of an ampere will flow through its secondary. One hundred milliamperes will in most cases be adequate to power the quiescent demands of the switchgear control.
Large capacitors on the order to 24,000 micro farads are used as power capacitors when the activating demands on the switchgear control are large. For every volt the power capacitor dips below the nominal bus voltage, the recovery time will be on the order of fifteen hundred cycles if 100 amperes of alternating current is flowing in the distribution line. If the power capacitor relied on the current transformer alone and was five volts below nominal operating level, more than a second will elapse before the control is restored. Considering that the normal current on a distribution line may fluctuate an order of magnitude in a twenty-four hour day the time to restoration may be in excess of ten seconds.
A particularly unfortunate situation can occur when the power capacitor is partially discharged in closing the distribution line switches and the control attempts to initiate a opening before the current transformers alone can recharge the power capacitors. Because the control lacks sufficient power to initiate the opening of the distribution line switches they never open. In many controls this results in the control latching in an attempt to open mode, since the current transformers lack the ability to supply the current required by the control in an active state. For this reason a primary source of power other than current transformers is customarily provided.